The present invention relates to voltage stabilizer devices and in particular to voltage stabilizers comprising monolithically integrated regulator circuits for use in motor vehicle applications.
Voltage stabilizers receive a voltage whose value is not defined and supply a voltage with a well defined and constant value or values.
Voltage stabilizers may be advantageously used as supply devices for other devices: i.e.--as a function of the load connected thereto, they in fact supply the current needed to keep the voltage supplied to this load constant.
At present, for reasons of compactness, ease of use and economic viability, integrated circuit electronic voltage stabilizers are tending to be constructed for all types of applications.
In general, the electrical magnitudes of voltage and current at the output terminals of these electronic voltage stabilizers are determined by an internal regulation circuit which is monolithically integrated and comprises a circuit feedback means which is connected to the output terminals and which is sensitive to the instantaneous values of these electrical magnitudes.
The lower limit of the correct operating range of an electronic voltage stabilizer is pinpointed by a parameter known in general in the technical literature by the term "drop-out", i.e. the difference between the minimum value of the input voltage required for the correct operation of the stabilizer and the value of the constant voltage which the stabilizer has to supply as output, which thus shows the voltage drop of the device.
Voltage stabilizers used in motor vehicle applications must satisfy particularly strict requirements as a result of operating conditions which entail major variations in temperature and humidity as well as considerable, and in some cases abrupt, variations in the supply voltage generated by the battery of the vehicle.
These stabilizers must therefore be extremely reliable, accurate and stable, while still being economically viable, and must in particular have a low drop-out since the supply voltage supplied by the battery of a vehicle may normally drop, during cold starting, from a typical 14.4 V at charge to approximately 6 V. Account must also be taken of the positive and negative voltage peaks with a maximum amplitude of up to 150 V which may be present on the supply line of a vehicle as a result of the switching transients of inductive loads (starter coils, relays, etc.) or of disconnections or breakages of electrical connection cables.
The monolithically integrated voltage regulator circuits most commonly used in voltage stabilizers for motor vehicle applications are those with a "series" type regulation, in which the output voltage is regulated to a constant value by a bipolar power transistor connected in series with an output terminal, the base of the transistor being appropriately controlled to cause it to conduct as a function of the load.
A suitably dimensioned power transistor can also withstand, without drawbacks, positive voltage peaks having a high amplitude and may thus continue to ensure the regulation of the output voltage.
Negative peaks in the input voltage could, however, cause the transistor to be cut off, with interruptions, albeit brief, in the supply to the user circuits connected to the voltage stabilizer, with serious drawbacks when these comprise memories and integrated logic circuits which require a constant supply.
For this reason voltage stabilizers comprising "series" type regulation circuits also comprise a capacitor and an input diode, which are not integrated, in order to maintain, during very short negative transients in the input voltage, an adequate supply to the power transistor.
FIGS. 1 and 2 of the drawings respectively show diagrams of a known voltage stabilizer with "series" type regulation by means of a PNP transistor and a known voltage stabilizer with "series" type regulation by means of an NPN power transistor.
The circuit diagram of FIG. 1 comprises a bipolar PNP transistor T having its emitter terminal connected to the cathode of a diode D whose anode forms an input terminal IN and to a first terminal of a capacitor C whose second terminal is connected to ground.
The collector terminal of the transistor T forms an output terminal OUT.
The base terminal of the transistor T is connected to the output terminal of a differential amplifier A whose non-inverting input is connected via a first resistor R.sub.1 to the terminal OUT and, via a second resistor R.sub.2, to ground.
The inverting input of the amplifier is connected to a voltage reference V.sub.R.
The part of the diagram of FIG. 1 which shows the voltage regulator circuit which can be monolithically integrated has been enclosed in a rectangular block shown by dashed lines.
In the circuit diagram of FIG. 2 the PNP transistor T is replaced by a bipolar NPN transistor T.sub.1. The output terminal of the differential amplifier A is not in this case connected directly to the base terminal of the transistor T.sub.1 but to the base terminal of a bipolar PNP transistor T.sub.2.
The emitter and collector terminals of the transistor T.sub.2 are respectively connected to the collector terminal and to the base terminal of the transistor T.sub.1.
All the other components of the diagram are identical to those of FIG. 1.
In both cases the capacitor C is charged via the diode D to the typical values of the battery voltage less the voltage drop at the diode, during normal charging conditions of the battery.
During the negative voltage transients, however, the diode D prevents the discharge of the capacitor C via the input terminal with the result that this capacitor can be discharged only via the transistor of the regulation circuit, enabling its conduction during the transient.
The two types of voltage stabilizer will now be compared, calculating their drop-out.
In the case of the stabilizer of FIG. 1 comprising a PNP power transistor, the drop-out is: EQU V.sub.DROP =V.sub.D +V.sub.CEsat
in which V.sub.D is the voltage drop across the diode D when it is conducting and V.sub.CEsat is the collector-emitter voltage of the transistor T when it is saturated.
In the case, however, of the stabilizer of FIG. 2 comprising an NPN power transistor, it is: EQU V.sub.DROP =V.sub.D +V.sub.CEsat +V.sub.BE
in which V.sub.D is again the voltage drop at the input diode, V.sub.CEsat is the collector-emitter voltage of the transistor T.sub.2 when it is saturated and V.sub.BE is the base-emitter voltage of the transistor T.sub.1 when it is conducting.
It is thus possible to construct a voltage stabilizer with minimal drop-out using, as shown in FIG. 1, a regulation circuit comprising a PNP power transistor.
A voltage stabilizer of the type shown in FIG. 2 is, however, economically advantageous, since by using an NPN power transistor it is possible to achieve an overall occupation of integration area of the regulation circuit which is lower than that which can be achieved with a PNP power transistor.
It should be noted that the with the voltage stabilizers examined above, with equal current supplied to the load, it is generally necessary to insert an external capacitor between the output terminal OUT and ground in order to stabilize the regulation loop during operation.
If use is made of an NPN power transistor, this provides a regulation loop with a gain which is lower than that which can be achieved with a PNP power transistor so that it is possible to use an output capacitor having a lower capacitance which is thus less costly.
Since the production level of devices for use with motor vehicles is very high, these economic advantages are fairly substantial.